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QLDPhysicsQuick questions
Unit 4: Revolutions in modern physics
Quick questions on Quantum theory: photons, the photoelectric effect and atomic spectra (QCE Physics Unit 4)
9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is threshold frequency?Show answer
This is the minimum frequency at which the photoelectric effect occurs.
What is stopping voltage?Show answer
The retarding voltage that stops all photoelectrons is the stopping voltage:
What is emission spectra?Show answer
Hot, low-density gas emits photons during electron transitions to lower levels. Bright lines on dark background, each line a specific wavelength.
What is absorption spectra?Show answer
Continuous light passes through cool gas; specific wavelengths are absorbed. Dark lines on bright continuous spectrum.
What is confusing photon energy with electron kinetic energy?Show answer
$E_{\text{photon}} = hf$ is fixed by the light. $E_{k,\max} = hf - \phi$ depends on both the photon and the metal.
What is forgetting the negative sign on bound energy levels?Show answer
$E_n$ is negative. The photon emitted has positive energy $E_i - E_f$, where both are negative and $E_i > E_f$ algebraically (less negative).
What is treating intensity as energy per photon?Show answer
Intensity is photons per second per area. Energy per photon is fixed by frequency.
What is applying $hc = 1240$ eV nm without unit check?Show answer
This works only when $\lambda$ is in nm and $E$ is in eV. Mixed units fail.
What is confusing emission and absorption directions?Show answer
Emission: higher to lower (energy out). Absorption: lower to higher (energy in).